Presentation on theme: "Now not all nuclei are “MRI active”.. Which of the following could produce an MRI image? 1H1H 11 C 13 N 18 F 19 F 31 P Only those with an odd number of."— Presentation transcript:
Now not all nuclei are “MRI active”.. Which of the following could produce an MRI image? 1H1H 11 C 13 N 18 F 19 F 31 P Only those with an odd number of protons and neutrons. Which isotopes at the right are radioactive?
The MRI signal is generated by receiving radiofrequency photons that return to their lower energy state. Does an MRI scanner produce radiation?
A hydrogen atom (whether bound in water or lipid) acts as a small magnet due to the spinning of the positively charged _______. proton
Protons from what compounds comprise an MRI signal? What percentage of your body is composed of water? What percentage of your body is composed of fat? A) 40%-50%, B) 50%-60%, C) 60%-70%, D) 70%-80% DescriptionWomenMen Essential fat10–12%2–4% Athletes14–20%6–13% Fitness21–24%14–17% Acceptable25–31%18–25% Overweight32-41%26-37% Obese42%+38%+
Typical Magnetic Field Map of a Clinical 3T MRI What effects will be felt by a pacemaker, credit cards, earrings, IPAD or cell phone?
The MRI scanner is always on!! A magnetic field is present 24/7!!
MRI Safety Implants and foreign bodies Projectile or missile effect Radio frequency energy Peripheral nerve stimulation (PNS) Acoustic noise Cryogens Contrast agents Pregnancy Claustrophobia and discomfort -“Cheap” Earrings - Tattoo Ink > Rock concert @ the gardens. - “Quench” - Nephrogenic Systemic Fibrosis - No X-rays/Gd crosses placenta.
How does resonance come into play in MRI? A tuning fork produces sound waves at a single frequency that may be detected by objects that are of lengths related to multiples of the wavelength. A typical tuning fork produces a frequency of 400 Hertz, while a scan from Sackler was actually resonating at 127, 503, 172 Hertz.
Larmor Equation: =Precessional Frequency = Gyromagnetic Ratio =Magnetic Field Strength (42.57 MHz/Tesla * 3.0 Tesla = 127.5 MHz) What field strength does my favorite FM Classic Rock station transmit at? Radio waves are transmitted at an angle of 90˚ into the body at the Larmor frequency. This imparts energy to the nuclei to achieve “resonance” The additional energy in turn rotates the nuclei out of alignment with the main field.
X Y Z Coil 3.0 Tesla GE MRI Scanner
“Magneto” Faraday’s Law of Induction states that a voltage is created by a changing magnetic flux. (1831) Was it easier back then to get a law named after you?
How do the motion of these two objects differ? “Rotation” vs. “Precession” It is the precession of the nuclei that creates the changing magnetic field needed to produce a signal.
What kind of signal is actually received by the scanner? The frequency & phase information in time from the Free Induction Decay “FID” are transformed into the frequency domain. (NMR 1946) A Fourier series can represent any function as a sum of sines and cosines. (1822)
H 2 O (4.7ppm) Lipids CH 2 (1.3ppm) Lipids CH 3 (0.9ppm) Typical NMR signal after Fourier transformation. Can you identify the peaks? How about concentration?
Where were all of these metabolic peaks hiding? What price is paid in detecting these signals?
Damadian’s Design for a Clinical MRI Scanner - 1974
Basic MRI Hardware Block Diagram How many of you have had an MRI? What’s it like?
20 dB 30 dB 40 dB 50 dB 60 dB 70 dB 80 dB Ticking watch Quiet whisper Refrigerator hum Rainfall Sewing machine Washing machine Alarm clock (two feet away) 85 dB 95 dB 100 dB 105 dB 110 dB 120 dB 130 dB Average traffic MRI Blow dryer, subway train Power mower, chainsaw Screaming child Rock concert, thunderclap Jackhammer, jet plane (100 feet away) How loud is loud? Fast imaging sequences such as EPI/Spiral used in functional neuroimaging (fMRI) can play upwards of 100+ decibels inside the bore of the scanner.
So how do we get spatial information? Back to the Larmor equation.. Magnetic Field Strength Position i.e. 1 Gauss will increase the frequency by 4.3kHz. Typical gradient strengths are 2-5 Gauss/cm.
What would the frequency difference be between two objects that are separated by 3cm? B = 42.57E6 Hz/Tesla B = G z * z = 0.01 T/m * 0.03 m = 12,771 Hertz
Conventional 3-Axis MRI Gradient Coil Diagram
Slice Selection 1 st step is to excite a single slice instead of all space! Frequency To excite a thickness z use: G Z To excite off axis use: where = G Z
How thin a slice could an MRI scanner produce? i.e. Could we perform in-vivo pathology scans? Slice Selection
Now that we have selectively excited a specific slice in space, we then must localize a specific xy-plane. With what pattern is MRI data generally acquired? Why would you choose one over the other?
Spatial encoding in x is called “Frequency Encoding”. The frequency of the signal ~ position on the x-axis. x = FOV x /N x = 1/( / G x x ) e.g. A standard brain scan uses a 24 cm FOV and a 512x512 matrix size on our 3T magnet. This gives an in-plane resolution of 0.47mm/pixel. RBW = N x / x = 1 / T e.g. A 15.63kHz RBW and G x = 0.3 G/cm would then apply the x-gradient for 32.8 ms to get a single line of image “k-space”.
Spatial encoding in y is called “Phase Encoding”. The phase of the signal ~ position on the y-axis. y = FOV y /N pe = =1/(2 / G yr y ) The phase of a signal is given by: t To acquire the next line in “k-space”, an additional phase ( G y y) is applied for a time t. This is repeated until the entire image space is covered. It is standard for the time to be fixed and the gradient amplitude to increase/decrease.
Why is a Fourier Transform used? Application of pulses in the “time” domain are transformed into the MRI “frequency” domain.
K-space vs. Image Space FT http://www.leedscmr.org/images/mritoy.jpg FT
http://www.radinfonet.com/cme/mistretta/traveler1.htm#part1 k-space Contribution to Image Properties Center = contrast Periphery = resolution
Voila’ - Spin Echo Images
How does an MRI scanner differ from a CT scanner? 1)Radiation, 2) Soft-Tissue Contrast The intensity on a CT scan is directly related to what? How much energy does MRI impart? E MRI =h( B 0 =0.3 eV vs. E CT ~ 25keV CT T1 T2
T1W GM=950ms WM=600ms T2W GM=100ms WM=80ms Image Weighting in MRI – * Learning Point *